Experimental analysis of latent thermal energy release through active power control techniques for domestic hot water generation

Abstract

This doctoral dissertation has been presented in the form of thesis by publication. In the last two decades, the world has experienced significant growth in energy consumption. In addition, the residential sector accounts for approximately one-third of the total energy consumption, with around 70% used for water heating and building heating. The growth of solar domestic hot water systems coupled with solar collectors is particularly notable. However, despite its growth, there are still challenges to overcome. The main drawback is the intrinsic intermittency of solar energy. Therefore, energy storage is essential for integrating intermittent renewable energy sources. One of the solutions employed in recent years to replace traditional storage methods is Phase Change Materials (PCMs). However, they present some issues to consider. One of the most important is the low thermal conductivity, which mainly affects the energy release process. This doctoral thesis proposes a novel solution that seeks to eliminate the formation of the solidified PCM layer on heat transfer surfaces using scraping techniques. The Scraped Surface Heat Exchanger (SSHE) designed, built, and studied in this work allows for an increase in the heat transfer rate with nearly constant heat flux, making it suitable for domestic hot wáter generation. Increases between 62% and 95% in the energy release rate have been obtained compared to a conventional exchanger (without scraping). Furthermore, a parametric study has been conducted to assess the influence of different operating parameters. It demonstrated the existence of a critical scraping frequency, beyond which increasing the frequency does not result in significant improvements in energy extraction. From this parametric study, a correlation was obtained that allows predicting solidification time as a function of the involved dimensionless operating parameters (Peclet, Stefan, Fourier numbers, and scraping frequency). Additionally, the long-term stability of the PCM has been evaluated. This was done after designing an accelerated procedure to obtain samples with different numbers of thermal and scraping cycles, allowing the evaluation of the effect of the shear stresses, the thermal cycles, and their combined effects separately. The analyses performed have shown that neither termal nor scraping cycles have a significant impact on the thermo-physical properties of the studied PCM.